Pleochroic anthraquinone dyes

ABSTRACT

An anthraquinone dye from water-solubilizing and ionic substituents of the general formula: ##STR1## wherein Q is NH 2 , OH, alkyl, aryl, NO 2  or halo; 
     n is 0 to 4; 
     each X independently is H, --SR, --NZ 1  Z 2  or Q; 
     each R independently is alkyl, aryl or cycloalkyl; 
     Z 1  &amp; Z 2  are independently H, alkyl, cycloalkyl or aryl; 
     provided that there are at least two different --SR groups and that where only two X&#39;s represent --SR groups at least one R is aryl and the two --SR groups are in the 1 &amp; 5 or the 1 &amp; 8 positions. 
     In addition to the normal textile applications the dye is suitable for use in the coloration of liquid crystals for electro-optic displays.

This application is a continuation-in-part of copending U.S. Application Ser. No. 352,058, filed Feb. 24, 1982.

This invention relates to anthraquinone dyes and their preparation.

According to the present invention there is provided an anthraquinone dye free from water-solubilising and ionic substituents of the general formula: ##STR2## wherein Q is NH₂, OH, alkyl, aryl, NO₂ or halo;

n is 0 to 4;

each X independently is H, --SR, --NZ₁ Z₂ or Q;

each R independently is alkyl, aryl or cycloalkyl; and

Z₁ & Z₂ are independently H, alkyl, cycloalkyl or aryl; provided that there are at least two different --SR groups and, where only two X's represent --SR groups, at least one R is aryl and the two --SR groups are in the 1 & 5 or the 1 & 8 positions.

The anthraquinone dye of Formula I is useful, in the form of a dispersion, for the colouration of synthetic fibre textile materials.

In addition to this more conventional dyestuff application, the anthraquinone dye of the present invention exhibits pleochroism and is particularly suitable for use in certain electro-optic displays containing liquid crystal materials.

Electro-optic display devices of certain known types depend upon liquid crystal properties displayed by certain compounds known as liquid crystal compounds or materials which display phases intermediate between the fully ordered crystalline state and the fully disorded liquid state, apart from certain temporary short range ordering that is present in most liquid phases.

Broadly speaking there are two principal types of liquid crystal phase, the smectic mesophase in which the molecular ordering is of a substantially lamellar type and the nematic mesophase in which the ordering is substantially linear. Included, sometimes as a sub-claim of the nematic mesophase and sometimes classified as a separate mesophase, is the cholesteric mesophase. This last type has a helical order arising from the presence of a chiral or optically active centre in the molecular composition of the material and this helical order is superimposed upon the linear order of the nematic mesophase.

Liquid crystal materials have the property of imposing their own ordering upon other molecules incorporated in the materials and having an appropriate molecular configuration or shape. This property is the basis of guest-host devices in which a "host" liquid crystal material has its orientation controlled by the application of electrical or magnetic fields and in turn imposes its order upon "guest" molecules of, for example, pleochroic dyes. These are dyes whose absorption properties vary with the orientation direction of the electric vector of light incident upon them relative to their own molecular alignment. A suitable pleochroic dye usually has an elongated rod-like molecule which absorbs relatiely little light passing along its longitudinal axis but has a maximum absorption of light having its electric vector oriented along the longitudinal axis of the molecule.

Such dye molecules when placed in a liquid crystal material can adopt an orientation which follows the molecular orientation given to the crystal material and, taking advantage of this, two broad classes of guest-host devices are possible based respectively upon a nematic (or Freedericksz effect) device and secondly upon a cholesteric to nematic phase-change device.

In a nematic device the liquid crystal material is originally oriented by known treatment of the inner surfaces of the container, e.g. glass plates bearing device electrodes, containing the liquid crystal material. This orientation is changed by application of an electric field between the device electrodes. The guest dye material also changes its orientation resulting in a change in absorption of light passing along the axis of the electrical field giving a switchable electro-optical display.

In a cholesteric-to-nematic phase change device the host liquid crystal material has a positive dielectric anisotropy and is, or includes, an optically active compound which causes the material to exhibit a cholesteric mesophase having a long-pitch helical ordering of short range (the "focal conic" state). In the "off" state the device scatters incident light because the ordering is only short range. However, when the device is switched on, the electrical field applied across the material imposes a linear nematic order parallel to the electric field which results in orientation of any guest dye molecules also parallel to the electrical field and provides minimum absorption in that direction. Thus in the "on" state the device is less scattering and a switchable display is obtained between the "on" and "off" states. The dye enhances the contrast between the two states.

In order to provide maximum contrast between the two states it is important that the guest molecules adopt as closely as possible the time averaged orientation of the host, however this is achieved only to a limited degree because of random thermal fluctuations. The degree to which the orientation varies from the ideal is measured by a quantity known as the order parameter S which is given by the following equation:

    S=1/2(3 cos.sup.2 θ-1)

where cos² θ is a time averaged term and θ is the instantaneous angular orientation of the molecules with respect to the time averaged orientation of the host molecules. The determination of the value of the order parameter S is well understood in the art, see for example the paper, "A new absorptive mode reflective liquid crystal display device", by D. L. White and G. N. Taylor in the Journal of Applied Physics, 1974, 45, pages 4178 to 4723.

For perfect orientation the order parameter S is one (that is θ is zero) and pleochroic dyes for use in guest-host devices should have an order parameter in the liquid crystal host as near one as possible but they must also have adequate chemical, photochemical and electrochemical stability, e.g. stability when exposed to atmospheric contaminants, to electric fields (as in device operation) and to ultra-violet radiation. They should not be ionic or have any ionisable character and should also have sufficient solubility in the host materials although the concentrations of guest pleochroic dye required for the desired effect are generally quite small (ca 10% wt. or below).

Preferred compounds within the scope of Formula I are those of the following formulae: ##STR3## wherein R¹ is aryl, R², Y¹ to Y³ and T¹ to T⁴ are each independently alkyl, aryl or cycloalkyl and each X¹ is independently H or --NZ₁ Z₂ and Q, Z₁ and Z₂ are as hereinbefore defined, provided that all of the radicals represented by R¹ and R², or by Y¹ to Y³ or by T¹ to T⁴ are not identical.

The alkyl group represented by R, R², Z₁, Z₂, any one of Y¹ to Y³ or any one of T¹ to T⁴, which may be branched or straight chain, desirably contains up to 20 carbon atoms but is preferably a lower alkyl group and more preferably contains from 1 to 4 carbon atoms. The cycloalkyl group represented by the same symbols preferably contains from 4 to 8 carbon atoms and is more preferably a cyclohexyl group. The aryl group represented by R, R¹, R², Z₁, Z₂, any one of Y¹ to Y³ or any one of T¹ to T⁴ prferably contains up to 15 carbon atoms and is more preferably a mono- or bi-carbocyclic aryl group such as phenyl or naphthyl.

The alkyl group represented by Q, which may be branched or straight chain, preferably contains from 1 to 20 carbon atoms and is more preferably lower alkyl. The aryl group represented by Q preferably contains up to 15 carbon atoms and is more preferably monocarbocyclic aryl, such as phenyl.

The alkyl, aryl and cycloalkyl radicals represented by R, R¹, R², Z₁, Z₂, Y¹ to Y³, T¹ to T⁴, and Q may be substituted by non-ionic groups. Phenyl groups are preferably substituted in the para (4-) position with respect to the linkage to the N or S atom or to the anthraquinone nucleus although, in the case of methylphenylthio substituents, a product containing a 3-methylphenylthio group is usually more soluble than the same product containing a 4-methylphenylthio group. Preferred substituent groups for the alkyl radicals are lower alkoxy, preferably C₁ to C₄ alkoxy, halogeno, preferably chloro, or aryl, containing up to 15 carbon atoms, preferably mono-cyclic aryl, such as phenyl. Preferred substituent groups for the aryl radicals, e.g. those represented by R³, R⁴ and T⁵ to T¹⁰ below, and cycloalkyl radicals are alkyl and alkoxy, containing up to 20 carbon atoms, preferably lower alkyl and lower alkoxy and more preferably C₁ to C₄ alkyl and alkoxy, halogeno, preferably chloro, C₄ to C₈ cycloalkyl, preferably cyclohexyl, and aryl, containing up to 15 carbon atoms, preferably monocyclic aryl, such as phenyl.

Throughout this specification the expressions "lower alkyl" and "lower alkoxy" mean alkyl and alkoxy radicals containing from 1 to 10 carbon atoms.

A preferred class of yellow to red dyes of Formula I are those in which n is 0 and two, three or four of the groups represented by X are --SR groups, the remainder being --NZ₁ Z₂ or, more preferably H. A preferred class of red to blue dyes according to Formula I are those in which 2 X's are --SR groups and 2 X's are groups represented by Q, especially OH or NH₂ groups. It is further preferred in this latter class of dyes that n is 1 or 2 and that each Q represents an alkyl radical containing up to 20 carbon atoms in positions adjacent to the OH or NH₂ groups.

It is generally preferred that at least one and more preferably all of the --SR groups are arylthio groups, and more preferably are phenylthio groups. The difference between the two aryl groups may occur in the aromatic nucleus but preferably occurs in the nature and/or position of a substituent on the aromatic nucleus. Where there are only two --SR groups they are preferably in the 1 and 5 positions. Another preferred group of dyes contains at least one arylthio group and at least one cycloalkylthio group.

The unsymmetrical di-, tri- and tetra(substituted thio)anthraquinones are generally more soluble in liquid crystal materials than the equivalent symmetrical di-, tri- and tetra(substituted thio)anthraquinones such as are disclosed in United Kingdom Patent Specification No. 2,043,097A and United Kingdom Patent Specification No. 2,082,196A. In this specification the term "unsymmetrical" used in relation to poly(substituted thio)anthraquinones indicates that there are at least two different substituted thio groups in the molecule whereas the term "symmetrical" in the same context indicates that all the substituted thio groups in the molecule are identical. The present invention does however include compositions comprising a mixture of symmetrical and unsymmetrical poly(substituted thio)anthraquinones, as well as the pure unsymmetrical compounds themselves and mixtures thereof. It is however preferred that the compositions comprise at least 50% by weight of the unsymmetrical poly(substituted thio)anthraquinones and more preferably at least 75% by weight of these unsymmetrical compounds. The unsymmetrical compounds can be obtained in a pure form, i.e. free from the closely related symmetrical compounds by chromatographic separation procedures, especially by preparative or high pressure liquid chromatography. The main advantage of the pure unsymmetrical compounds over compositions containing mixtures of these with the symmetrical compounds is the generally greater solubility of the former which is of importance in the contrast and stability, over a wide temperature range, of the liquid crystal display.

Solubility is, however, only one of the factors affecting the achievement of good contrast in a liquid crystal display, another factor is the extinction coefficient of the dye in the liquid crystal material. One useful indication of the ability of a dye to give good contrast is the product of the molar extinction coefficient and the solubility (in moles/liter). For good commercial performance it is desirable that solutions of dyes in liquid crystal compositions for use in electronic displays should have a value for this product which is at least 500 cm⁻¹ and preferably at least 750 cm⁻¹. As the molar extinction coefficient for a dye does not vary significantly from one liquid crystal material to another, these values for the above defined product are a direct indication of the desired and preferred minimum solubilities of a particular dye in any liquid crystal material. Thus for a dye having a molar extinction coefficient of 11,000 cm².moles⁻¹ the solubility should preferably be at least 4.5×10⁻² moles/liter and more preferably at least 6.8×10⁻² moles/liter. In fact the yellow dye of Example 1 has this value for the extinction coefficient and a molecular weight of 480 so that the solubility (wt %) for this dye should preferably be at least 2.2% and more preferably be at least 3.3%; the actual solubility of this dye in the liquid crystal material E43 (see Example 1) is 8.6%. For a dye having a molar extinction coefficient of 16,000 cm².moles⁻¹ the solubility should preferably be at least 3.1×10⁻² moles/liter and more preferably at least 4.6×10⁻² moles/liter. In fact the red dye of Example 1 has this value for the extinction coefficient and a molecular weight of 650 so that the solubility of this dye (wt %) would preferably be at least 2% and more preferably at least 3%; the actual solubility of this dye in the liquid crystal material E43 is almost 15%. For a dye having a molar extinction coefficient of 20,000 cm².moles⁻¹ the solubility is preferably be at least 2.5×10⁻² moles/liter and more preferably at least 3.8×10⁻² moles/liter. In fact the violet dye of Example 21 has this value for the extinction coefficient and a molecular weight of 590 so that the solubility should preferably be at least 1.5% and more preferably at least 2.2%; the actual solubility of this dye in the liquid crystal material E43 is 4.6%.

Dyes prepared for use in liquid crystal displays should preferably be as pure as possible in terms of their freedom from inorganic and other ionisable materials which can interfere with the operation of the display or from materials which are radiation sensitive and decompose within the display during operation. The dyes should also preferably be free from non- or inferior pleochroic materials, such as starting materials, intermediates and by-products, which do not contribute to the perceived contrast of the display or which are less soluble in the liquid crystal material. To obtain the dyes in a pure form, i.e. substantially free from interfering or deleterious matter, it is generally desirable to submit them to repeated recrystallisations from organics solvents such as chloroform and/or chromatographic separation procedures.

The anthraquinone dyes of the present invention have very high stability in liquid crystal media and high order parameters, generally greater than 0.7, in both positive and negative anisotropic liquid crystal materials. The di(substituted thio)anthraquinones are of particular value because stable yellow dyes with high order parameters and good solubility have not heretofore been obtained.

As the addition of any dye to a liquid crystal material raises the viscosity of the latter and thus tends to increase the response time of a display in which it is incorporated, it is desirable to use as little dye as possible. In this respect the present dyes are of particular value because many of them have high extinction coefficients and thus only small quantities, generally less than about 10%, are required in the liquid crystal material.

The anthraquinone dyes of the present invention may be mixed with other pleochroic dyes to produce intermediate shades and neutral shades, such as greys and blacks, the latter being especially suitable for use in black-on-white electronic displays.

A preferred class of yellow dyes, containing two different --SR groups, has the general formula: ##STR4## wherein R¹ is aryl containing up to 15 carbon atoms and R² is any of the groups represented by R above provided that R¹ and R² are not identical.

Radicals which may be represented by R² include C₁ to C₂₀ alkyl, up to C₁₅ aryl and C₄ to C₈ cycloalkyl which may carry one or more non-ionic substituents such as those mentioned above. When R² is an aryl radical, it must be different from the aryl radical represented by R¹. This difference may occur in the aromatic nucleus or in the nature and/or position of a substituent thereon. Useful compounds of this type include those of the formula: ##STR5## wherein R³ represents hydrogen or a non-ionic substituent, R⁴ represents a non-ionic substituent and R³ differs from R⁴ in constitution and/or in its position on the phenyl radical.

Particular mention may be made of compounds of the formula: ##STR6## wherein R³ and R⁴ have the meanings given above and R³ differs in constitution from R⁴. Especially useful compounds of this type include those in which R³ is H and R⁴ is lower alkyl, especially t-butyl, or phenyl.

Other especially preferred compounds according to Formula V are those in which R² is cyclohexyl, and particularly where R¹ is phenyl or 4-t-butylphenyl.

Dyes of Formula V may be prepared by reacting one equivalent of the thiol, R¹ SH, and one equivalent of the thiol, R² SH, with one equivalent of a 1,5-dihalogenoanthraquinone. The reaction is conveniently carried out in a solvent, for example dimethylformamide, and in the presence of an acid-binding agent, for example potassium carbonate.

The product of this reaction is a statistical mixture containing the compound of Formula V as major component together with the symmetrical compounds of the formulae: ##STR7##

Compounds of Formula V may be prepared in a purer form by reacting a 1-halogeno-5-nitroanthraquinone with one equivalent of the first thiol (R¹ SH) under mild conditions and then reacting the product with one equivalent of the second thiol (R² SH) at a higher temperature.

In U.K. Patent Application GB 2,043,097A, liquid crystal compositions are disclosed which can contain symmetrical dyes of the formula: ##STR8## wherein T is hydrogen or a non-ionic substituent. Dyes of this class, such as 1,5-bis(phenylthio)anthraquinone, have high order parameters and adequate stability but, in general, their solubility in the liquid crystal materials is insufficient for practical purposes giving somewhat weak colour effects. Mixtures of two or more compounds of Formula X also have low solubilities in the liquid crystal materials.

It is surprising that the unsymmetrical dyes of Formula V have significantly higher solubilities in liquid crystal materials than the symmetrical dyes of Formulae VIII, IX and X. Although the pure unsymmetrical dyes also have higher solubilities than those mixtures mentioned above which contain the symmetrical compounds of Formulae VIII and IX as impurities, the mixtures of unsymmetrical and symmetrical dyes have sufficiently higher solubilities than the pure symmetrical dyes to give them commercial utility.

A preferred class of red dyes, containing four --SR groups, not more than three of which are identical, has the general formula: ##STR9## wherein each of T¹, T², T³ and T⁴ may be any of the groups represented by R above, provided not more than three are identical. Preferred values for T¹ to T⁴ are phenyl substituted phenyl and cyclohexyl.

Particularly useful compounds of Formula XI are those in which each of T¹, T², T³ and T⁴ is an aryl radical of which not more than three are identical. Differences between these radicals may occur in the aromatic nucleus or in the nature and/or position of the substituents thereon. Useful compounds of this type include those of the formula: ##STR10## wherein each of T⁵, T⁶, T⁷ and T⁸ independently represents hydrogen or a non-ionic substituent, as hereinbefore described, at least one of T⁵, T⁶, T⁷ and T⁸ being different from the other in either constitution or position.

Particular mention may be made of compounds of the formula: ##STR11## wherein T⁹ represents hydrogen or a non-ionic substituent, T¹⁰ represents a non-ionic substituent, T⁹ and T¹⁰ not being identical, and m is from 1 to 3. Preferred compounds of this type include those in which T⁹ is H, T¹⁰ is phenyl, cyclohexyl or lower alkyl, especially t-butyl, and m is 1.

Dyes of Formula XI may be prepared by reacting a 1,4,5,8-tetrahalogenoanthraquinone with one equivalent of each of the thiols: T¹ SH, T² SH, T³ SH, T⁴ SH, not more than three of T¹, T², T³ and T⁴ being identical. The reaction is conveniently carried out in a solvent, for example dimethylformamide, and in the presence of an acid-binding agent, for example potassium carbonate.

Thus, compounds of Formula XIII may be prepared by reacting the tetrahalogenoanthraquinone with m equivalents of a thiol of formula: ##STR12## and 4-m equivalents of a thiol of formula: ##STR13##

The product of these reactions is a statistical mixture in which compounds of Formula XI predominate together with lesser amounts of compounds in which T¹, T², T³ and T⁴ are identical.

In UK Patent Application No. 8,024,797, liquid crystal compositions have been proposed which contain symmetrical dyes of the formula: ##STR14## wherein T represents an optionally substituted alkyl or aryl radical, Q represents halogen, hydroxy, amino, alkylamino, dialkylamino, arylamino, nitro, alkyl or aryl and y represents an integer of 0 to 4. Particular mention is made of 1,4,5,8-tetrakis(phenylthio) anthraquinone. These symmetrical tetra(substituted thio)anthraquinones have high order parameters and adequate stability but their solubility in liquid crystal materials is generally lower than is desirable for practical purposes.

It is surprising that the unsymmetrical dyes of Formula XI have significantly higher solubilities in liquid crystal materials than the symmetrical dyes of Formula XIV.

The preferred group of orange dyes of Formula III may be prepared by reacting two different substituted thiols with a trichloroanthraquinone, in the presence of an acid binding agent, to give a composition containing mainly the unsymmetrical compounds together with a smaller proportion of the symmetrical compounds. Alternatively one thiol may be reacted with a nitrodichloroanthraquinone under mild conditions to give a (substituted thiol)dichloroanthraquinone and the second thiol reacted with the (substituted thio)dichloroanthraquinone intermediate under more severe conditions to give a single unsymmetrical tri(substituted thio)anthraquinone with only minor quantities of the symmetrical products.

The --NZ₁ Z₂ groups and the groups represented by Q are preferably introduced prior to the --SR groups by known methods for introducing such groups, e.g. by reacting an amino compound HNZ₁ Z₂ with the appropriate chloranthraquinone.

Examples of other suitable compounds of Formula I are:

1-phenylthio-5-(naphth-2-ylthio)anthraquinone,

1-phenylthio-5-(dodecylthio)anthraquinone,

1-phenylthio-5-(4-nonylphenylthio)anthraquinone,

1-(phenylthio)-4,5,8-tri(cyclohexylthio)anthraquinone

1-(phenylthio)-4,5,8-tri(4-phenylphenylthio)anthraquinone,

1-(phenylthio)-4,5,8-tri(4-t-butylphenylthio)anthraquinone,

1-phenylthio-4-anilino-5-(4-methylphenylthio)anthraquinone,

1-phenylthio-4,8-di(butylamino)-5-butylthio-anthraquinone,

and

1-phenylthio-5-cyclopentylthioanthraquinone.

Another preferred class of dyes within the scope of the present invention as defined in Formula I, are those in which n is 1 or 2 two of the groups represented by X are --SR and the remainder are OH or NH₂ and each Q is C₁₋₂₀ -alkyl, and more especially those in which n is 2, two of X are different phenylthio groups and the other two of X are OH. Within this latter class it is further preferred that the OH groups are in the 1 and 5 positions or the 1 and 8 positions and that the C₁₋₂₀ -alkyl groups are adjacent to the OH groups.

This latter class is included in yet another preferred class of dyes which may be represented by the formula: ##STR15## wherein: P is OH or NHZ³ ; each Q¹ is C₁₋₂₀ -alkyl; n is 1 or 2; and X² and X³ are selected from OH, NHZ³ and --SR, provided that when X² is OH or NHZ³, X³ is --SR and when X³ is OH or NHZ³, X² is --SR; R is as hereinbefore defined and Z³ is H or C₁₋₄ -alkyl. Within the class defined by Formula XV the two groups represented by R may be the same but are preferably different.

Preferred dyes within the class represented by Formula XV are defined by the formula: ##STR16## wherein: L & M are H or CH₂ K¹ ; X⁴ & X⁵ are OH or SR² provided that when X⁴ is OH X⁵ is SR², L is CH₂ K² and M is H; and when X⁵ is OH X⁴ is SR², L is H and M is CH₂ K² ; K¹ & K² are C₁₋₂₀ -alkyl; and R¹ & R² are as hereinbefore defined.

Within the class of dyes represented by Formula XVI it is preferred that K¹ and K² are identical and that they represent C₃₋₁₂ -alkyl which may be branched or straight chain. An especially preferred dye from this class having particularly good solubility and a high order parameter is 1,8-dihydroxy-2,7-di-n-heptyl-5-(4-t-butylphenylthio)-4-phenylthioanthraquinone. Other useful dyes of this class include 1,8-dihydroxy-2,7-di-n-pentyl-4-(4-t-butylphenylthio) 5-phenylthioanthraquinone, 1,8-dihydroxy-2,7-di-n-dodecyl-4-phenylthio-5-(4-t-butylphenylthio)anthraquinone and 1,5-dihydroxy-2,6-di(3,5,5,5-tetramethyl-n-pentyl)-4-phenylthio-8-(4-t-butylphenylthio)anthraquinone. The dyes of Formula XVI generally exhibit bluish-red shades and are generally characterised by good solubility and high order parameters in liquid crystal materials.

The dyes of Formula XVI may be prepared by the successive substitution of the nitro groups in a compound of the formula: ##STR17## wherein: L & M are H or CH₂ K¹ ; X⁶ & X⁷ are OH or NO₂ provided that when X⁶ is OH X⁷ is NO₂, L is CH₂ K² and M is H; and when X⁷ is OH X⁶ is NO₂, L is H and M is CH₂ K² ; and K¹ & K² are C₁₋₂₀ -alkyl, with two different substituted thio groups, --SR¹ and --SR². The dyes of Formula XV wherein both groups represented by R are the same may be prepared by the simultaneous or the successive substitution of both nitro groups by a single thio group --SR. A preferred process has two stages, the first comprising the reaction of a mercaptan, R¹ SH, with the compound of Formula XVII, in pyridine at a temperature around or below 30° C., to give an intermediate compound of the formula: ##STR18## wherein: L & M are H or CH₂ K¹ ; X⁶ & X⁷ are OH or NO₂ provided that when X⁶ is OH X⁷ is NO₂, L is CH₂ K² and M is H; and when X⁷ is OH X⁶ is NO₂, L is H and M is CH₂ K² ; K¹ & K² are C₁₋₂₀ -alkyl; and R¹ is as hereinbefore defined, and the second comprising the reaction of the intermediate compound of Formula XVIII with a second mercaptan, R² SH, at a higher temperature. The second stage, the reaction with the second mercaptan, is preferably performed in a polar solvent, more preferably a hydroxylic solvent such as an alkanol, e.g. isopropanol, and is also preferably performed above 60° C. and more preferably at the reflux temperature of the solvent.

The starting material for this process may be prepared by the reaction of quinizarin or anthrarufin with one or more aldehydes of the formula K¹ CHO and K² CHO in the presence of pyridine to give a 2,7-dialkylquinizarin or a 2,6-dialkylanthrarufin and nitrating this to give a 2,7-dialkyl-4,5-dinitroquinizarin or a 2,6-dialkyl-4,8-dinitro-anthrarufin of Formula XVII.

It has also been found that some of the intermediate compounds of Formula XVIII and the reduction products thereof, wherein the NO₂ group is converted into an NHZ³, Z³ being H or C₁₋₄ -alkyl, have good solubilities and/or order parameters in liquid crystal materials and according to a further feature of the present invention there is provided an anthraquinone dye of the formula: ##STR19## wherein: L & M are H or CH₂ K¹ ; X⁶ & X⁷ are OH, NO₂ or NHZ^(3h) provided that when X⁶ is OH, X⁷ is NO₂ or NHZ³, L is CH₂ K² and M is H; and when X⁷ is OH, X⁶ is NO₂ or NHZ³, L is H and M is CH₂ K² ; K¹ & K² are C₁₋₂₀ -alkyl; and and SR¹ is as hereinbefore defined.

The compound of Formula XIX wherein X⁶ or X⁷ is NHZ³ may be prepared from the compounds of the same formula in which X⁶ or X⁷ represent NO₂ by reduction with a mixture of sodium sulphide and sulphur in an aqueous medium optionally followed by alkylation.

Preferred compounds within the class represented by Formula XIX are those in which X⁶ is OH and X⁷ is NH₂ and those in which X⁷ is OH and X⁶ is NH₂. Examples of compounds of Formula XIX are 1,8-dihydroxy-4-phenylthio-5-amino-2,7-di(3,5,5,5-tetramethyl-n-pentyl)anthraquinone, 1,5-dihydroxy-4-phenyl-2,6-di(3,5,5,5-tetramethyl-n-pentyl) 8-aminoanthraquinone and 1,5-dihydroxy-4-phenyl-8-nitro-2,6-di(3,5,5,5-tetramethyl-n-pentyl)anthraquinone.

The invention is further illustrated by the following Examples in which all parts and percentages are by weight unless otherwise indicated and the letters "AQ" are used to represent anthraquinone.

EXAMPLE 1

1,4,5,8-tetrachloroanthraquinone (12 g) was added to a stirred mixture of dimethylformamide (100 ml), potassium carbonate (12 g), thiophenol (10.4 ml) and 4-t-butylphenylthiol (14.8 ml). The reaction mixture was heated to 130°-140° C. and maintained at that temperature for 5 hours. After cooling overnight, the reaction product was filtered and the filter cake was washed several times with a 50:50 mixture of ethanol and 2 N sodium hydroxide solution. The washed product was then washed with very dilute acetic acid, washed acid free with water, dried at 50° C. in vacuo, dissolved in the minimum amount of boiling toluene, filtered, reprecipitated by the addition of methanol, filtered of, washed with methanol and dried at 50° C.

The product contained the following substituted anthraquinones (AQ):

                  TABLE 1                                                          ______________________________________                                         Component               Wt. %                                                  ______________________________________                                         tetrakis-(phenylthio)AQ  1%                                                    tris-(phenylthio)-t-butylphenylthio-AQ                                                                 19%                                                    bis-(phenylthio)-bis-(t-butylphenylthio)AQ                                                             65%                                                    tris-(t-butylphenylthio)-phenylthio-AQ                                                                 11%                                                    tetrakis-(t-butylphenylthio)AQ                                                                          4%                                                    ______________________________________                                    

The solubility of the product was measured in liquid crystal material E43 marketed by BDH Chemicals Limited of Broom Road, Poole, Dorset, England. E43 contains the compounds: ##STR20##

The solubility of the product in E43 at 20° C. is 14.7%. For comparison, the solubilities of pure 1,4,5,8-tetrakis(phenylthio)anthraquinone and pure 1,4,5,8-tetrakis(4-t-butylphenylthio)anthraquinone in the same material are, respectively, <1% and 1.8%.

EXAMPLE 2

The procedure of Example 1 was repeated except that the 4-t-butylphenylthiol was replaced by the equivalent quantity of 4-methylphenylthiol.

EXAMPLE 3

Thiophenol (2.97 g), 4-phenylphenylthiol (4.38 g) and potassium carbonate (3.73 g) were stirred in 23 ml dimethylformamide(DMF) at 120° C. for 1 hour and cooled to ambient temperature. 1,4,5,8-Tetrachloroanthraquinone (3.46 g) was added, the mixture held at 120° C. for 4 hours and then cooled to ambient. After the addition of 20 ml ethanol the mixture was worked up by the following procedure. After filtration and washing with a 50:50 mixture of 2 N NaOH and ethanol the crude product was slurried in 30 ml of the same solvent mixture, stirred for 30 minutes, filtered, washed successively with the caustic soda/ethanol solvent and water and dried at 80° C.

The work-up product was boiled in 30 ml chloroform, filtered cold and the filtrate added dropwise to 250 ml petroleum ether (100-120). After stirring for 1 hour the precipitate was filtered, washed with 40-60 petroleum ether and dried at 80° C. The product contained about 90% unsymmetrical tetra(substitutedthio)anthraquinones, carrying both phenylthio and 4-phenylphenylthio groups, the remainder being symmetrical tetra(substitutedthio)anthraquinones, carrying either phenylthio or 4-phenylphenylthio groups.

EXAMPLE 4

1.98 G thiophenol, 2.07 g of cyclohexylthio and 2.02 g of KOH in 30 ml ethanol were stirred at reflux for 1 hour and cooled to ambient temperature. 1,4,5,8,-Tetrachloro-AQ (2.31 g) was added and the mixture stirred at reflux temperature for 16 hours before cooling to room temperature and working up according to the work-up procedure of Example 3. The crude solid was dissolved in 100 ml hot chloroform, screened and drowned into 300 ml methanol over a 5 minute period. The drownout mass was stirred for 30 minutes and the solid filtered, washed with methanol and dried at 80° C. (yield 2.8 g). The main constituents of the product are unsymmetrical tetra(substituted thio)AQs, carrying cyclohexylthio and phenylthio groups in the 1,4,5 and 8 positions.

EXAMPLE 5

The procedure of Example 4 was repeated except for the replacement of the cyclohexylthiol by the equivalent weight of 4-methoxyphenylthiol. The main constituents of the product are unsymmetrical tetra(substituted thio)AQs carrying phenylthio and 4-methoxyphenylthio groups in the 1, 4, 5 and 8 positions.

EXAMPLE 6

1,5-Dichloroanthraquinone (12 g) was added to a stirred mixture of dimethylformamide (100 ml), potassium carbonate (8.0 g), thiophenol (thiol 1:7.0 ml) and 4-t-butylphenylthiol (thiol 2:8.0 ml). The reaction mixture was heated to 130°-140° C. and maintained at that temperature for 5 hours. After cooling overnight, the reaction product was filtered and the filter cake was washed several times with a 50:50 mixture of ethanol and 2 N sodium hydroxide solution. The washed product was then washed with very dilute acetic acid, washed acid-free with water, dried at 50° C. in vacuo, dissolved in the minimum amount of boiling toluene, filtered, re-precipitated by the addition of methanol, filtered off, washed with methanol and dried at 50° C.

The product contained 55% of 1-(4-t-butylphenylthio)-5-phenylthioanthraquinone, the remainder being the two symmetrical 1,5-disubstituted anthraquinones.

The following 4 analogous products were prepared by the method of Example 6 using the equivalent quantities of the appropriately substituted thiols in place of the thiophenol and the 4-t-butylphenylthiol used in Example 6.

EXAMPLE 7

Thiol 1: thiophenol

Thiol 2: 3-methylphenylthiol

Main product: 1-phenylthio-5(3-methylphenylthio)AQ

% in compn: 48%

EXAMPLE 8

Thiol 1: 3-methylphenylthiol

Thiol 2: 4-t-butylphenylthiol

Main product: 1(3-methylphenylthio)-5(4-t-butylphenylthio)AQ

% in compn: 64%

EXAMPLE 9

Thiol 1: 3-methylphenylthiol

Thiol 2: 4-methylphenylthiol

Main product: 1(3-methylphenylthio)-5(4-methylphenylthio)AQ

EXAMPLE 10

Thiol 1: thiophenol

Thiol 2: 4-methylphenylthiol

Main product: 1-phenylthio-5(4-methylphenylthio)AQ

EXAMPLE 11

A mixture of 73.7 g 4-t-butylphenylthiol (thiol 2) 25 g potassium hydroxide, and 640 ml ethanol was heated at reflux temperature for 1 hour and cooled to room temperature. To this mixture was added 86.5 g 1-phenylthio-5-chloroanthraquinone, over a period of 5 minutes, the mixture heated to reflux and held there for 16 hours. At the end of the reaction period the mixture was cooled to room temperature, the solid product filtered off and washed with 200 ml 50% 2 N NaOH/ethanol. The crude product was reslurried into a further 200 ml of the same mixed solvent, stirred for 30 minutes, refiltered, washed successively with 100 ml 50% 2 N NaOH/ethanol and 100 ml water and dried at 80° C.

The product was extracted with 2 liters 15:1 mixture of 100-120 petroleum ether and chloroform at reflux for 48 hours following which the chloroform was distilled off. The slurry in petroleum ether was cooled and filtered after 24 hours at room temperature, washed with 100 ml 40-60 petroleum ether and dried at 80° C.

After repeated recrystallisation from methanol the product, containing about 88% 1-phenylthio-5-(4-t-butylphenylthio)anthraquinone, had a melting point of 241° C.

The 1-phenylthio-5-chloroanthraquinone used above was prepared by adding 72.5 g 1-nitro-5-chloroanthraquinone and 500 ml ethanol to a mixture of 78 g of thiophenol (thiol 1), 32 g potassium carbonate and 125 ml dimethylformamide (previously refluxed at 120° C. for 2 hours and cooled to room temperature) and heating to 40° C. for 16 hours. After cooling and standing for 48 hours the solid product was filtered, washed with 1 liter 40% aqueous ethanol, reslurried in 500 ml of the same solvent, stirred for 30 minutes, filtered and washed successively with 200 ml 40% aqueous ethanol and 1 liter water. The product was reslurried in 500 ml water for 30 minutes, filtered, washed with water and finally with 100 ml ethanol and dried at 80° C.

Table 2 compares the solubilities of the dyes of Examples 6 and 11, with those of the equivalent symmetrical dyes, in the liquid crystal material E43 (see Example 1) at 20° C.

                  TABLE 2                                                          ______________________________________                                                                       Solubility                                       No.   Dye                     (wt. %)                                          ______________________________________                                         1     1,5-bis(phenylthio)anthraquinone                                                                       1.7                                              2     1,5-bis(4-t-butylphenylthio)anthraquinone                                                              2.0                                              3     Physical 50/50 mixture of 1 and 2 above                                                                <2.0                                             4     1-phenylthio-5-(4-t-butylphenylthio)                                                                   8.6                                                    anthraquinone (product of Example 11)                                    5     Chemical mixture of 1, 2 and 4 above                                                                   3.2                                                    (product of Example 6)                                                   ______________________________________                                    

It is clear from the solubility data listed in Table 2 that the unsymmetrical product, either alone or in admixture with the symmetrical equivalents, has a considerably higher solubility in the liquid crystal medium than either of the symmetrical products or their mixture. This higher solubility is not only completely unexpected but of considerable commercial significance in view of the demand for liquid crystal displays which are stable over a wide temperature range from below 0° C. to around 100° C.

The following six analogous compositions were prepared according to the process of Example 11 using the equivalent quantities of the appropriately substituted thiols in place of the thiols used in Example 11.

EXAMPLE 12

Thiol 1: thiophenol

Thiol 2: 3-methylphenylthiol

Main product: 1-phenylthio-5(3-methylphenylthio)AQ

% in compn.: 81%

M.P.: 227° C.

EXAMPLE 13

Thiol 1: thiophenol

Thiol 2: 4-methylphenylthiol

Main product: 1-phenylthio-5(4-methylphenylthio)AQ

% in compn.: 83%

M.P.: 269° C.

EXAMPLE 14

Thiol 1: thiophenol

Thiol 2: n-butylthiol

Main product: 1-phenylthio-5(n-butylthio)AQ

% of compn.: 98%

M.P.: 163° C.

EXAMPLE 15

Thiol 1: thiophenol

Thiol 2: 4-phenylphenylthiol

Main product: 1-phenylthio-5(4-phenylphenylthio)AQ

% in compn.: 91%

EXAMPLE 16

Thiol 1: thiophenol

Thiol 2: cyclohexylthiol

Main product: 1-phenylthio-5-cyclohexylthio-AQ

EXAMPLE 17

Thiol 1: thiophenol

Thiol 2: 4-methoxyphenylthiol

Main Product: 1-phenylthio-5-(4-methoxyphenylthio)AQ

EXAMPLE 18

A solution of 1.98 g of thiophenol, 2.99 g of 4-t-butylphenylthiol and 2.48 g of potassium carbonate in 20 ml of DMF was heated to 115° C. and stirred at this temperature for 1 hour. To the solution was added 3.28 g of 1,5-dinitro-4,8-diamino-AQ and the mixture stirred for 8 hours at 115° C. It was then cooled to room temperature, filtered, washed successively with DMF, water and ethanol and dried to give 1.7 g of crude product.

The crude product was purified by screening in hot chloroform and, after cooling, passage through a silica-packed chromatography column. The column was eluted with chloroform and the main blue band was collected and evaporated to dryness. The dry material was slurry washed with 20 ml methanol, filtered, washed with methanol and dried at 80° C. to give 0.2 g of purified composition containing 63% 1-phenylthio-5(4-t-butylphenylthio)-4,8-diamino-AQ.

The 1,5-dintro-4,8-diamino-AQ was prepared from 1,5-diamino-AQ-bis-N,N-dimethylformidinium chloride by the method of Example 2 of U.K. Patent Specification No. 1,041,528 and the 1,5-diamino-AQ-bis-N,N-dimethylformidinium chloride was made from 1,5-diamino-AQ by the method of Example 2 of German Patent Specification No. 1132931.

EXAMPLE 19

Thiophenol(1.98 g), 3-methylphenylthiol(2.33 g) and potassium carbonate(2.02 g) were stirred at 115° C. in 15 ml DMF for 1 hour and cooled to room temperature. To this mixture were added 4.24 g of 1,5-dibromo-4,8-di(methylamino)AQ over 5 minutes and the mixture reheated to 120° C. and stirred at this temperature for 16 hours. After cooling to ambient temperature and the addition of 25 ml ethanol the crude material was worked up as in Example 3 to yield 4.35 g. The worked up material was dissolved in chloroform and passed through a silica-packed chromatography column eluting with chloroform and collecting the main red band. After evaporation of the chloroform, washing with methanol and drying at 80° C. the yield of purified material was 4.09 g containing 48.9% of 1-phenylthio-5(4-t-butyl phenylthio)-4,8-di(methylamino)AQ, the remainder being the two symmetrical di(substituted thio)AQs.

EXAMPLE 20

A mixture of 1.24 g of thiophenol, 1.87 g of 4-t-butylphenylthiol and 1.56 g of potassium carbonate in 10 ml DMF were stirred at 120° C. for 1 hour and cooled to ambient temperature. To the cooled mixture were added 1.73 g of 1-nitro-5,8-dicloro-4-anilino-AQ and the mixture stirred at 120° C. for 5 hours. After cooling to ambient temperature and adding 27 ml ethanol and 40 ml water the solid was filtered, washed with water and ethanol and dried to yield 3.55 g of crude material.

The crude material was dissolved in chloroform and passed through a silica-packed chromatography column, eluting with chloroform and collecting the main orange band. After evaporating the solvent, 30 ml methanol were added and the solid material filtered, washed with methanol and dried at 50° C. (yield 2.95 g). The purified material contained 65% unsymmetrical tri(substituted thio)AQs, the remainder being mainly the two symmetrical tri(substituted thio)AQs.

The 1-nitro-4-anilino-5,8-dichloro-AQ was prepared according to the following method: A mixture of 7.83 g of aniline and 10 g of 1,4,5-trichloro-8-nitro anthraquinone in 75 ml 2-ethoxyethanol was heated at 120° C. for 18 hours. The product was filtered, washed with ethanol, slurried in 50 ml ethanol and refiltered. It was then stirred into 50 ml 2 N HCl, filtered, washed acid free with water and dried at 80° C. The final stage of purification consited of stirring the dry material in 50 ml chloroform for 20 minutes, filtering, washing with chloroform and drying at 80° C. to yield 5.9 g of product (m.p. 216°-7° C.) having a purity of 90.5%, by chromatography.

EXAMPLE 21

A mixture of 1.48 g thiophenol, 2.24 g 4-t-butylphenylthiol and 1.5 g KOH were refluxed in 15 ml ethanol for 1 hour and cooled to ambient temperature. To the cooled mixture were added 1.5 g of 1,4,5-trichloro-AQ and the mixture refluxed for 16 hours. After cooling to ambient temperature the mixture was worked up as described in Example 3. The 2.3 g of worked up material contained 62% unsymmetrical tri(substituted thio)AQs, i.e. di(phenylthio)-(4-t-butylphenylthio)AQ and phenylthio-di(4-t-butylphenylthio)AQ, the remainder being mainly the two symmetrical tri(substituted thio)AQs.

EXAMPLE 22

A mixture of 0.127 g of thiophenol and 0.065 g KOH in 15 ml ethanol was refluxed for 1 hour and cooled to ambient temperature. To the cooled mixture was added 0.66 g of 1,5-di(4-t-butylphenylthio)-4-chloro-AQ and the mixture refluxed for 16 hours before cooling to ambient temperature. After working up as decribed in Example 3 the yield of dry material was 0.6 g. This material contained 78% of 1,5-di(4-t-butylphenylthio)-4-phenylthio-AQ.

The 1,5-di(4-t-butylphenylthio)-4-chloro-AQ was prepared as follows: A mixture of 2.98 g of 4-t-butylphenylthiol and 0.51 g KOH in 10 ml ethanol was refluxed for 1 hour and cooled to ambient temperature. To this was added 1.56 g of 1,4,5-trichlor-AQ and the mixture stirred at 40° C. for 16 hours before cooling to ambient temperature. After working up the crude material as described in Example 3 it was further purified by dissolving in 100 ml 60-80 petroleum ether, screening and passage through a silica-packed chromatography column, eluting with the same solvent and collecting the middle orange band. After evaporation of the solvent, washing with methanol and drying the yield of purified material was 0.9 g.

EXAMPLE 23 (a) Preparation of 1,5-dihydroxy-2,6-dinonyl-4-phenylthio-8-nitroanthraquinone

A mixture of 1.1 ml (0.01 moles) of thiophenol and 5.8 g of 1,5-dihydroxy-2,6-dinonyl-4,8-dinitro-AQ (0.01 moles) in 50 ml pyridine was stirred for 30 minutes at room temperature and then drowned out into 100 ml water. Concentrated HCl (75 ml) was added and the precipitated solid was filtered, washed with water and dried to yield 6 g of crude material. A portion of this (3 g) was recrystallised from 100-120 petroleum ether to yield 1.3 g of the compound 1,5-dihydroxy-2,6-dinonyl-4-phenylthio-8-nitroanthraquinone (structure confirmed by mass spectrometry).

(b) Preparation of 1,5-dihydroxy-2,6-dinonyl-4-phenylthio-8-(t-butylphenylthio)anthraquinone

A mixture of 0.65 g of the product from Example 23(a) and 0.2 g of 4-t-butylphenylthiol in 10 ml pyridine was stirred at 90°-5° C. for 16 hours and then drowned out into 50 ml of dilute HCl. The precipitated solid was filtered, washed successively with water and methanol and dried. It was then recrystallised from 40-60 petroleum ether to yield 0.1 g of product consisting mainly of 1,5-dihydroxy-2,6-dinonyl-4-phenylthio-8-(t-butylphenylthio)-AQ (structure confirmed by mass spectrometry).

(c) Preparation of 1,5-dihydroxy-2,6-dinonyl-4-phenylthio-8-aminoanthraquinone

A solution of 10 g of Na₂ S.9H₂ O and 2 g of sulphur in 20 ml of water was added to 3.0 g of the product of Example 23(a) in 100 ml of water and 50 ml of pyridine and heated to 90° C. After 1 hour at 90° C. the pyridine was evaporated and the remainder cooled to 60° C. The oil which separated was extracted into 100-120 petroleum ether and the water removed (by Dean & Stark). The solution was purified by passage through a silica gel column and a small quantity of HCl added to precipitate a solid. The solid (1.2 g) was dried at 70° C. and the structure confirmed by mass spectral and NMR analysis.

(d) Preparation of 1,8-dihydroxy-2,7-di-nonyl-4-phenylthio-5-aminoanthraquinone

The procedure of Example 23(c) was repeated using 3.0 g of 1,8-dihydroxy-2,6-dinonyl-4-phenylthio-5-nitro-AQ in place of the product of Example 23(a). The structure of the product was confirmed by mass spectrometry. The 1,8-dihydroxy-2,6-dinonyl-4-phenylthio-5-nitro-AQ used as starting material was prepared according to the method of Example 23(a) using 5.8 g of 1,8-dihydroxy-2,7-dinonyl-4,5-dintro-AQ in place of the 1,5-dihydroxy-2,6-dinonyl-4,8-dinitro-AQ.

The 1,5-dihydroxy-2,6-dinonyl-4,8-dinitro-AQ and the 1,8-dihydroxy-2,7-dinonyl-4,5-dinitro-AQ mentioned in Examples 23(a) and (d) were made according to the method described in U.K. Patent Specification No. 2038809A by reacting 1,5-dihydroxy-AQ and 1,8-dihydroxy-AQ, respectively, with nonanal and nitrating the di-nonyl derivatives. The alkyl radical in the nonanal had the formula:

    --CH.sub.2.CH.sub.2.CH(CH.sub.3).CH.sub.2.CH(CH.sub.3).sub.3

(e) Preparation of 1,5-dihydroxy-2,6-dinonyl-4,8-diphenylthio-AQ

The procedure of Example 23(b) was repeated using the equivalent amount of thiophenol in place of the 4-t-butylphenylthiol.

(f) Preparation of 1,8-dihydroxy-2,7-dinonyl-4,5-diphenylthio-AQ

The procedure of Example 23(e) was repeated using the equivalent amount of 1,8-dihydroxy-2,7-dinonyl-4-phenylthio-5-nitro-AQ in place of the 1,5-dihydroxy-2,6-dinonyl-4-phenylthio-8-nitro-AQ.

EXAMPLE 24 (a) Preparation of 1,8-dihydroxy-2,7-didocyl-4-phenylthio-5-nitroanthraquinone

A mixture of 6.6 g of 1,8-dihydroxy-2,7-didodecyl-4,5-dinitro-AQ, 1.1 g of thiophenol and 100 ml pyridine were stirred mixed at ambient temperature for 2 hours and then drowned out into 200 ml water. The precipitated solid was filtered, washed successively with water and methanol and dried to yield 4.5 g of crude material. After recrystallisation from 100-120 petroleum ether the product was 1,8-dihydroxy-2,7-didodecyl-4-phenylthio-5-nitro-AQ (4.15 g: structure confirmed by mass spectrometry).

(b) Preparation of 1,8-dihydroxy-2,7-didocyl-4-phenylthio 5-(4-t-butylphenylthio)anthraquinone

A portion of the product of Example 24(a) (3.7 g) was added to a solution of 0.83 g of 4-t-butylphenylthiol and 0.3 g KOH in 50 ml iso-propanol at 65° C. After heating the mixture at the reflux (83° C.) for 3 hours it was cooled to ambient temperature, filtered, washed successively with isopropanol and methanol, dried and recrystallised from 100-120 petroleum ether. The product (2.75 g) was essentially 1,8-dihydroxy-2,7-didodecyl-4-phenylthio-5-(4-t-butylphenylthio)-AQ.

The 1,8-dihydroxy-2,7-didodecyl-4,5-dinitro-AQ was made according to the method described in U.K. Patent Specification No. 2038809A by reacting 1,8-dihydroxy-AQ with n-dodecanal and nitrating the di-n-dodecyl derivative.

EXAMPLE 25 (a) Preparation of 2,7-Di-n-pentylchrysazin

Chrysazin (120 g) was stirred into a solution of 120 g of sodium hydroxide in a mixture of water (1500 ml) and pyridine (220 ml) at 55° C. Sodium hydrosulphite (200 g) was added to the mixture which was stirred under N₂ for 30 minutes before addition of a further 100 g of sodium hydrosulphite. The reaction mixture was heated to 80° C., over 30 minutes during which period was added, dropwise, n-pentanal (142 g) and the mixture stirred for 2 hours at 80° C. still under N₂. It was then cooled to room temperature, poured into an acidified ice/water mixture (ice/water: 4 liters and 35% HCl: 700 ml), in which it was stirred for 2 hours, filtered and the filter cake washed with water. The tarry cake was slurried in 400 ml of ethyl acetate, filtered, washed with methanol and pulled until damp-dry. This paste was recrystallised from ethyl acetate to give 83 g of dry product (yield 45%) melting at 127° C. Elemental analysis gave C=76.1% (75.8%) and H=7.5% (7.4%).

(b) Preparation of 2,7-Di-n-pentyl-4,5-dinitrochrysazin

2,7-Dipentylchrysazin (56 g) from Example 25(a) was stirred in 800 ml of 98% sulphuric acid at room temperature for 30 minutes and then cooled to 0° C. To the cooled mixture was added, dropwise over a period of 30 minutes while maintaining the temperature <10° C., 160 ml of an anhydrous nitration acid, comprising 33% nitric acid and 67% sulphuric acid, and the mixture stirred for 2 hours at <10° C. The mixture was then poured into 4 liters of ice/water, stirred at room temperature for 30 minutes, filtered, washed acid-free and dried to give 50.5 g (yield 73%) of dry product melting at 140° C. Elemental analysis gave C=58.8% (61.3%), H=5.4% (5.5%) and N=5.5% (6.0%).

(c) Preparation of 2,7-Di-n-pentyl-4-phenylthio-5-nitrochrysazin

To a mixture of 9.4 g of 2,7-di-n-pentyl-4,5-dinitrochrysazin from Example 25(b) in 200 ml of pyridine at 20° C. there was added a solution of 2.2 ml of thiophenol in 50 ml of pyridine. The colour of the mixture gradually changed from orange-brown to red and after 2 hours at 20° C. was drowned into water, filtered, washed successively with water and methanol and dried to yield 9.5 g of product (88% theory). The structure was confirmed by mass spectrometry and elemental analysis, C=67.5 (67.5) H=5.8 (5.8) S=6.7 (6.0) and N=3.0 (2.7). This product had a molar extinction coefficient of 12,541 at 540 nm in chloroform.

(d) Preparation of 2,7-di-n-pentyl-4-phenylthio-5-(4-t-butylphenylthio)chrysazin

To a mixture of 1.7 ml of 4-t-butylphenylmercaptan and 0.4 g of KOH in 50 ml of isopropanol at 60° C. was added 3.7 g of the product of Example 25(c) and the temperature raised to 80° C. for 1.5 hour. It was then cooled to room temperature, filtered, washed successively with isopropanol and methanol and dried to give 3.5 g of product. After recrystallisation from petroleum ether, the structure was confirmed by mass spectrometry and elemental analysis, C=73.0 (73.6) H=6.5 (6.8) S=9.1 (9.8). This product had a molar extinction coefficient of 19,183 at 592 nm in chloroform.

EXAMPLE 26 (a) Preparation of 2,7-Di-n-heptyl-chrysazin

The process of Example 25(a) was repeated except for the replacement of the 142 g of n-pentanal with 171 g of n-heptanal. The quantity of dry product was 103 g (yield 47.6%) which melted at 116°-118° C. Elemental analysis gave C=76.1% (75.8%) and H=7.5% (7.4%).

(b) Preparation of 2,7-Di-n-heptyl-4,5-dinitrochrysazin

The process of Example 25(b) was repeated using 87.2 g of 2,7-di-n-heptylchrysazin in place of the 56 g of 2,7-di-n-pentylchrysazin and increasing the quantity of the other ingredients by 25%. The quantity of dry product was 50.5 g (yield 69.1%) melting at 140° C. Elemental analysis gave C=58.8% (61.3%), H=5.4% (5.5%) and N=5.5% (6.0%).

(c) Preparation of 2,7-Di-n-heptyl-4-phenylthio-5-nitrochrysazin

The process of Example 25(c) was repeated using 9.6 g of 2,7-n-heptyl-4,5-dinitrochrysazin in place of the 9.4 g of 2,7-di-n-pentyl-4,5-dinitrochrysazin. The structure was confirmed by mass spectrometry. This product had a molar extinction coefficient of 12,095 at 540 nm in chloroform.

(d) Preparation of 2,7-Di-n-heptyl-4-phenylthio-5-(4-t-butylphenylthio)chrysazin

The process of Example 25(d) was repeated using 3.8 g of 2,7-di-n-heptyl-4-phenylthio-5-nitrochrysazin in place of the 3.7 g of 2,7-di-n-pentyl-4-phenylthio-5-nitrochrysazin. After recrystallisation from petroleum ether, the structure was confirmed by mass spectrometry. This product had a molar extinction coefficient of 19,650 at 592 nm in chloroform.

EXAMPLE 27

A sample of the product prepared according to the process described in Example 1 was passed through a high pressure liquid chromatography column packed with Partisil 5u (250×4.6 mm) using 99:1 hexane:acetonitrile as eluent at a flow rate of 2 ml/min and ambient temperature (20° C.). Detection was by means of a u.v. source (254 nm) and samples, corresponding to all the peaks in the chromatogram, were collected. The fractions collected are identified in Table 3, together with their properties in the liquid crystal medium, E43, at 20° C.

                  TABLE 3                                                          ______________________________________                                                              Solubility                                                                              Order   max                                      Fraction                                                                              Structure     (%)      Parameter                                                                              (nm)                                     ______________________________________                                         1      Tetrakis-X-AQ 1.8      0.78    550                                      2      Tri-X-mono-Y-AQ                                                                              10.7     0.78    550                                      3      Di-X-di-Y-AQ  <0.5     --      550                                      3a     Di-X-di-Y-AQ  1.0      0.78    550                                      4      Di-X-di-Y-AQ  2.9      0.77    550                                      5      Mono-X-tri-Y-AQ                                                                              1.0      0.78    550                                      6      Tetrakis-Y-AQ <1.0     0.78    550                                      ______________________________________                                    

In Table 3, X represents 4-t-butylphenylthio- and Y represents phenylthio-.

Fractions 3 and 3a had the same retention time and were separated by their different solubilities in the eluent.

EXAMPLE 28

A black mixture dye was made by mixing together 48 parts of the yellow dye of Example 11, 33 parts of the red dye of Example 1 and 19 parts of the pleochroic blue dye, 1,8-dihydroxy-4,5-diamino-2,7-di-isobutylanthraquinone. A 3.65% solution of this dye in Liquid Crystal Host 10 (see Table 5 below) is especially suitable for use in a 10⁻⁵ m reflective guest-host display with a single polariser.

EXAMPLE 29

A black mixture dye was made by mixing together 46 parts of the yellow dye of Example 11, 32.5 parts of the red dye of Example 1 and 21.5 parts of the pleochroic blue dye used in Example 26. A 6.5% solution of this dye in the liquid crystal mixture E43 is suitable for use in a phase change guest host display without a polariser.

The compounds described in the Examples have the properties identified in Table 4 as solutions in the liquid crystal medium, E43, at 20° C., unless otherwise stated.

                  TABLE 4                                                          ______________________________________                                                 Solubility     λmax                                                                             Order                                          Example (%)            (nm)     Parameter                                      ______________________________________                                          1      14.7           550      0.80                                            1      2.5 (-2° C.)                                                     2      1.0            550      0.80                                            3      3.0            50       0.77                                            4      4.7            550      0.75                                            6      3.2            464      0.80                                            7      3.3            464      0.78                                            8      2.4            464      0.78                                            9      2.5            464      0.80                                           10      2.9            464      0.78                                           11      8.6            464      0.79                                           11      4.4 (-2° C.)                                                    12      2.4            464      0.78                                           13      1.8            464      0.80                                           14      3.1            470      0.75                                           15      4.0            460      0.78                                           16      9.2            465      0.81                                           18      0.8            605      0.73                                           19      1.0            648      0.72                                           20      7.8            615      0.71                                           21      2.0            520      0.77                                           22      14.3           520      0.81                                           23(a)   2.8            565/610  0.72                                           23(b)   4.6            590      0.79                                           23(c)   5.8            565/610  0.73                                           23(d)   6.2            565/610  0.74                                           23(e)   2.0            590      0.77                                           23(f)   3.0            590      0.72                                           24(d)   3.6            590      0.83                                           25(d)   2.5            592      0.77                                           26(d)   7.2            592      0.78                                           ______________________________________                                    

The properties of some of the dyes described in the Examples were also measured in other liquid crystal media (hosts) at 20° C., unless otherwise stated, and the results are set out in Table 5.

                  TABLE 5                                                          ______________________________________                                                Dye of     Order     Solubility                                         Host   Example    Parameter (%)                                                ______________________________________                                         1      11         0.73                                                         1      6          0.72                                                         1      1          0.73                                                         2      11         0.79      4.0                                                2      6          0.80      2.0                                                2      1          0.80      4.6                                                3      11         0.81      2.5                                                3      6          0.81      1.5                                                3      1          0.79      ca 5.0                                             4      11         0.74                                                         4      1          0.75                                                         5      6          0.68      3.1                                                5      1          0.75      7.2                                                6      6          0.78      2.0                                                8      6          0.78                                                         9      11         0.81                                                         9      6          0.81                                                         9      1          0.80                                                         10     11         0.78      4.0 (-2° C.)                                10     1          0.78      1.1 (-2° C.)                                11     11         0.78      >5.0                                               11     11                   4.4 (-2° C.)                                11     1          0.80      >5.0                                               11     1                    2.2 (-2° C.)                                12     11         0.77      2.0                                                12     1          0.78      3.0                                                ______________________________________                                    

The liquid crystal materials referred to in Table 5 as Hosts 1 to 12 have the following compositions:

Host 1 is the liquid crystal material E7, available from BDH Chemicals Ltd, which includes the compounds:

4-cyano-4'-n-heptyl-biphenyl 25%

4-cyano-4'-n-pentyl-biphenyl 51%

4-cyano-4'-n-octyloxy-biphenyl 16%

4-cyano-4"-n-pentyl-terphenyl 8%

Host 2 is the liquid crystal material ZLI 1132, available from E Merck & Co, which includes the following compounds:

1-cyano-4-(n-propylcyclohexyl)benzene,

1-cyano-4-(n-pentylcyclohexyl)benzene,

1-cyano-4-(n-heptylcyclohexyl)benzene

and 4-cyano-4'-(n-pentylcyclohexyl)biphenyl

Host 3 is the liquid crystal material ZLI 1695, available from E Merck & Co, Darmstadt, West Germany. This has a clearing point (nematic to isotropic transition temperature) of 72° C.

Host 4 is a mixture of alkyl-cyanophenyl-dioxanes containing n-propyl, n-pentyl and n-heptyl chains.

Host 5 is the liquid crystal material ZLI 1565, available from E Merck & Co. This has a clearing point of 85° C.

Host 6 is the liquid crystal material ZLI 1624, available from E Merck & Co. This has a clearing point of 87° C.

Host 8 is the liquid crystal material RO-TN 430 available from Hoffmann La Roche Co. This has a clearing point of 69° C.

Host 9 is a mixed bicyclo-octane liquid crystal material containing the following compounds:

1-cyano-4[n-propyl-bicyclo(2,2,2)octyl]benzene 30%

1-cyano-4[n-pentyl-bicyclo(2,2,2)octyl]benzene 40%

1-cyano-4[n-heptyl-bicyclo(2,2,2)octyl]benzene 30%

Host 10 is a mixture of liquid crystal materials based mainly on cyanobiphenyls and available from BDH Chemicals Ltd under the name E61.

Host 11 is a mixture of liquid crystal materials based mainly on cyanobiphenyls and available from BDH Chemicals Ltd under the name E49.

Host 12 is a mixture of fluorobicyclo(2,2,2)octane esters having negative anisotropy and containing a mixture of two compounds of the following formula:

    1-alkyl-3-fluoro-4-[4-alkyl-bicyclo(2,2,2)octylcarbonyloxy]benzene

1 wherein both alkyl groups are pentyl 55%

2 wherein one is pentyl and one propyl 45%

The very high stability of the dyes of the present invention is illustrated in Table 6, which sets out the results of experiments to determine the effect of exposing solutions of the dyes in a liquid crystal medium (E43) to u.v. and visible radiation. Deterioration of the dye was detected by a decrease in the absortion of the solution at max. The times (in hours) quoted in the table are for a reduction of 10% from the initial value.

                  TABLE 6                                                          ______________________________________                                                        Time (hours) for 10%                                                           Reduction in Absorption                                                        Dye of:                                                         Radiation        Example 6 Example 1                                           ______________________________________                                         "Black" light    10,000    20,000                                              (U.V., 20° C.)                                                          Xenon arc (40° C.)                                                                        5,000     5,000                                              (U.V. + visible)                                                               ______________________________________                                     

I claim:
 1. An anthraquinone dye of the formula: ##STR21## wherein L & M are H or CH₂ K¹ ; X⁴ & X⁵ are OH or SR² provided that when X⁴ is OH X⁵ is SR², L is CH₂ K² and M is H; and when X⁵ is OH X⁴ is SR², M is CH₂ K² and L is H; K¹ & K² are C₁₋₂₀ -alkyl; R¹ is mono- or bi-cyclic aryl; and R² is different to R¹ and selected from C₁₋₂₀ -alkyl, mono- or bi-cyclic aryl and C₄₋₈ -cycloalkyl.
 2. An anthraquinone dye of the formula: ##STR22## wherein X² and X³ are selected from OH and --SR² provided that when X² is OH X³ is --SR² and when X³ is OH X² is --SR² ; each Q is C₁₋₂₀ -alkyl and adjacent to an OH group; and R¹ & R² are different and selected from alkyl, mono- or bi-cyclic aryl and cycloalkyl.
 3. An anthraquinone dye according to claim 2 or claim 1 wherein at least one of the alkyl, aryl and cycloalkyl radicals represented by R¹ and R², carries a non-ionic substituent.
 4. An anthraquinone dye according to claim 3 wherein the substituent on the aryl radical is selected from C₁₋₁₀ -alkyl, C₄₋₈ -cycloalkyl and phenyl.
 5. A composition comprising at least one unsymmetrical di(substituted thio)anthraquinone dye according to claim 2 or claim 1, in which the two substituted thio groups are different and at least one symmetrical di(substituted thio) anthraquinone dye, in which the two substituted thio groups are identical.
 6. A composition according to claim 5 containing at least 50%, by weight, of the unsymmetrical di(substituted thio)anthraquinone dye.
 7. An anthraquinone dye or composition according to claim 1 or claim 2 wherein the product of the molar extinction of the dye or composition and the solubility of the dye (in moles/liter) is at least 500 cm⁻¹.
 8. 1,8-Dihydroxy-2,7-di-n-heptyl-4-phenylthio-5-(4-t-butylphenylthio)anthraquinone. 